In recent years, immune mechanisms on mucosal membranes covering the respiratory apparatus, the digestive apparatus, the reproductive organs, and other organs have been gradually elucidated as the immune mechanism to protect an organism from infectious diseases such as influenza and AIDS. For example, an immune response derived from influenza virus infection is associated with an IgA antibody secreted on a mucosal membrane, an IgG antibody induced in blood to neutralize the virus, and cytotoxic T cells that lyse infected cells to interrupt virus transmission. Such a mucosal immune mechanism is an immune system present at the initial phase of the infection, and plays a key role in biophylaxis at the time of the infection or during the initial phase of the infection. Accordingly, a mucosal vaccine inducing an immune system preventing infection on a mucosal surface, which is the first barrier at portals of entry for a pathogen, is regarded as effective for various mucosal infections.
A mucosal vaccine induces production of a secretory IgA antibody in a mucosal tissue through transmucosal administration such as transnasal administration, and also induces production of an IgG antibody in a serum. Thus, the mucosal vaccine is capable of inducing immune responses in both the mucosal and systemic systems against a pathogen, and in addition, is superior to conventional injection administration in terms of operability, safety, and economic efficiency, and accordingly, is expected for clinical application as a novel vaccine, and has been developed.
On the other hand, since a mucosal vaccine for singly administering an antigen is not capable of inducing a sufficient immune response, it has been revealed that an adjuvant for a mucosal vaccine is indispensably used together in order to induce an effective immune response on the mucosal surface. Up to the present, many adjuvants for mucosal vaccines have been reported, and for example, bacterial endotoxins such as cholera toxin (CT) and heat-labile enterotoxin (LT) of enterotoxigenic E. coli, are known as representative adjuvants for activating mucosal immunity (Non-Patent Documents 1 and 2). However, it has been reported that clinical trials with LT transnasal administration caused facial nerve palsy (Bell's palsy), and hence, it is now regarded that there is a problem of safety in development of nasal adjuvants using toxins themselves such as CT and LT. Besides, MPL resulting from attenuation of the activity of endotoxin LPS, bacterial flagellar protein Flagellin (Patent Document 1), double-stranded RNA (poly(I:C)) (Patent Document 2) and the like, all of which are not toxins, have been studied as adjuvants for mucosal immunity activation, but since those candidates induce excessive inflammatory responses, they are also not satisfactory in terms of safety. In this manner, no effective and safe adjuvant for a mucosal vaccine has been put to practical use at present.
On the other hand, hemagglutinin (HA) and a nontoxic-nonhemagglutinin (NTNH) component bind to botulinum neurotoxin (NTX) produced by botulinum causing food poisoning, so as to form a large neurotoxin complex (progenitor toxin (PTX)) having a molecular weight of 300,000, 500,000, or 900,000. When botulism is caused, botulinum toxin blocks neuron transmission and leads to death, but taking advantage of the activity thereof, it is used as an effective neurotransmission inhibitor for medical purposes. For example, a botulinum toxin type A (BOTOX) complex is known to be used for treatment of blepharospasm, hemifacial spasm, spasmodic torticollis and heterotropia, and reduction of wrinkles. Among the neurotoxin complexes described above, non-toxic hemagglutinin (HA) is known to have functions of disrupting the barrier function of epithelial cells on the basolateral membrane side and transporting botulinum neurotoxins and macromolecules. When NTX and an albumin antigen are subcutaneously administered to a mouse in combination with HA, antigen-specific production of an antibody in blood is enhanced through IL-6 production (Non-Patent Document 3). While Patent Documents 3 and 4 describe the adjuvant activity of an HA subcomponent (HA1 or HA3) and use of a nucleic acid as a carrier for intracellular introduction, no protein complex composed of HA subcomponents (HA1, HA2 and HA3) is not discussed. The present inventors previously reported that HA acts on M cells in the epithelial cell layer of the Peyer's patch (M cells on the Peyer's patch), and that HA assists migration and entrance of a neurotoxin complex from the M cells to the basolateral side via transcytosis (Non-Patent Document 4). While the function of the neurotoxin complex (the HA to which a toxin component has been bound) to cross the intestinal epithelial barrier was investigated in the study described above, interaction of toxin-free HA with M cells and an adjuvant effect for delivering a vaccine for a mucosal infection have not yet been discussed. Besides, Non-Patent Document 5 describes M cell orientation of an HA complex, and Non-Patent Documents 6 and 7 describe involvement of HA2-HA3 in E-cadherin binding, but the function of the HA complex as a mucosal vaccine adjuvant is not discussed in any of these.